Useful tangential forces transmitted between two friction wheels depend on the properties of the surfaces in engagement with each other and the amount of contact force compressing the engaging surfaces together. For a given coefficient of friction between engaging surfaces and a fixed amount of contact force, there is a limit to the size of the tangential force that can be transmitted before slip occurs.
The engineering theory of the geometry, deformations and state of stress of two elastic bodies with curved surfaces in tangential contact is dealt with in "FORMULAS FOR STRESS AND STRAIN" by R. J. Roark, Chapter 13, McGraw-Hill, New York, 1954. The term "Hertzian Contact" used in the specification refers to the intimate surface contact zone between loaded traction rollers which follows the engineering theory referenced above. "Hertzian Zone" is the contact zone itself. Hertz was the mathematician who developed the theory of surface stresses in "H. Hertz: `GESAMMELTE WERKE`, Volume 1, Leipzig, 1895."
Fixed ratio friction wheel transmissions are of two types:
The first is where the friction wheels are arranged so as to be engaged with each other with a fixed predetermined contact force chosen to prevent slip at the maximum load. Unnecessarily large contact forces are imposed at lower transmitted loads resulting in lower life expectancy. PA1 The second type has the friction wheels arranged so that they are firmly engaged by contact forces that are developed by internal reactions to torque being transmitted. This type can be further classified as to whether the friction wheel bearings must carry very high contact forces found between the friction wheels or relatively low tengential forces delivering the useful work in the transmission. PA1 1. Contact forces between all traction rollers are torque responsive. Therefore, the tractive capability of the transmission to deliver torque automatically changes to exactly meet the load level demanded. This makes gross slippage of traction rollers nearly impossible under normal operating conditions. PA1 2. The design arrangement is simple and reflects in low manufacturing and assembly costs. PA1 3. High bearing loads are virtually eliminated since major contact forces between rollers are internally counter-balanced by the traction rollers and are not reacted out into bearings. PA1 4. Increase or reduction speed ratios can be obtained ranging from less than 2:1 to over 25:1. PA1 5. Extremely high speeds can be handled at relatively low surface velocities thus offering new design alternatives for high speed machinery. PA1 In general, the number of degrees of freedom possessed by a moving body relative to a fixed frame of reference is determined by the number of independent variables that are required to define its position. Of interest in the present invention and in prior art are those degrees of freedom present in the principle plane of rotation of the rollers which is that plane which is perpendicular to the roller axes. The only degrees of freedom possible in the principle plane of rotation in devices of the type under consideration are: one in rotation about the roller axis, and either, none, one or two, in translation of the geometric center of the roller from the fixed reference. "Geometric center", as used herein is defined as the center of a circle formed by the intersection of a plane and the traction surface of the roller with the plane intersecting the traction surface at a right angle to the roller axis. PA1 Only the degrees of freedom in translation are of specific interest here. For further clarification, consider the three cases shown in FIG. 6 in which graphic illustrations of degrees of freedom are made. PA1 It has been found that the actual dynamic operating geometry assumed by the rollers under load and speed is significantly different from that present in the static, unloaded state. The differences are not easily discovered except by careful analysis and testing. PA1 1. The sum of the degrees of freedom of movement of the geometric centers of the five traction rollers in their principle planes of rotation must be equal to either 8, 9 or 10; PA1 2. The five traction roller diameters and coordinate positions of their geometric centers must follow definite mathematical relationships heretofore not defined by prior art; and PA1 3. Traction contact stresses must be balanced at the various contact lines between the rollers according to definite mathematical equations if optimum design proportions and transmission performance are to be realized.
The present invention is of the second type where the bearings carry only low tangential forces.
Examples of various fixed ratio friction wheel transmissions are described in "MECHANICAL DESIGN AND SYSTEMS HANDBOOK" by Harold A. Rothbart, pp. 14-8 and 14-9, McGraw Hill, New York, 1964.
Essential to successful operation of the present and prior inventions of this type is that the traction roller elements be permitted sufficient freedom of movement so each roller can share in load carrying.
Advantages of these transmissions, if they can be made to work, include:
Numerous attempts have been made by past inventors in proposing design approaches that utilize traction roller arrangements resembling the present invention in order to gain some of these advantages. However, virtually all of the prior art has failed to take into account the adverse influence of elasticity of engineering materials used in traction roller drive designs on the actual operating geometry. Previous designs strongly suggest poor load sharing capability of the power rollers. When the traction rollers are not free to adjust position to accommodate the various elastic deformation effects inevitably induced by the transmitted load, they experience high wear, excessive creep, rapid heat generation, noise and poor speed and torque efficiency.
Since the key feature that differentiates the present invention from prior art is essentially the number of degrees of freedom permitted the five traction rollers, specific definitions and clarifications are hereby made:
There are very subtle, but vitally important reasons why consideration of the number of degrees of freedom is essential in the design of traction drives of the type described in the present invention:
Elastic deformations occur in the rollers, bearings, shafts, etc., such that the operating geometric centers of the rotating parts shift to new dynamic positions. Unless these displacements can be accommodated in a predictable fashion in the design of the device, it will experience wear, noise and other related operating problems. Undoubtedly inventors in prior art were faced with the dilemma of either accepting poor performance or building the devices so heavy and cumbersome to control deformations that they became impractical. Also, as components are made stiffer, they must also be made more accurate and, therefore, more costly. It is probably noteworthy that none of the devices proposed by the prior art has exhibited any significant commercial success to date.
One prior invention, Barske U.S. Pat. No. 3,380,312, requiring a total of not more and not less than 6.degree. of freedom, clearly shows that the two reaction rollers, in a five roller system, always have 0.degree. of freedom each and thus correspond to the FIG. 6a example. The inventor very clearly illustrates by diagram and by description when he is intending either 0, 1 or 2 degrees of freedom. His definitions of degrees of freedom are substantially in agreement with those in FIGS. 6a, b and c.
A more recent invention, Kraus U.S. Pat. No. 3,776,051, of a somewhat related roller arrangement involving six traction roller elements does not discuss essential degrees of freedom per se, except for the wedging roller; but does clearly illustrate by diagrams that the device, depending on interpretation, has either 4 or 5 degrees of freedom at most and will not care for the adverse effects of elastic deformation previously discussed.
A very early invention of related construction Dieterich U.S. Pat. No. 1,093,922 clearly shows a five roller device with an internal ring with 2.degree. of freedom, an intermediate roller with 1.degree., an inner sun roller with 0.degree. and the remaining two intermediate rollers with either 1 or 2 degrees each. Therefore, the total degrees of freedom of movement for this early system are either 5 or 7, but definitely not 8, 9 or 10.